Particles shall refer to small-grain materials having a grain size (particle size) in the nanometer and micrometer range. This shall include both powders, in general up to particle sizes of a few μm, and granules, particle sizes in the micrometer range.
Surfaces treated or coated with particles are known from various sectors and can have very different structures. For example, surface protection and surface upgrading by powder coating are known. In order to achieve surfaces of high hardness or wear resistance, surfaces are alloyed with an additive applied in pulverulent form.
For production of lithium ion batteries, porous active materials having a layer thickness of 20 μm up to a few hundred micrometers are required for the electrodes, which serve for the intercalation of the lithium ions. Gas diffusion layers (GDL), which are used for production of fuel cells, have a high and simultaneously finely distributed gas permeability. It may be necessary here to process the gas diffusion layer in the form of a microporous layer in order to increase the gas distribution capacity, electrical conductivity and/or water repellency thereof.
Various methods are known for application of powder to a surface to be treated or coated, depending on the surface or layer thickness to be achieved. Frequently, a powder is mixed with a binder by wet-chemical or mechanical means and applied to the surface, for example, by spraying, slot-die coating, screen printing or what is called spin-coating and dried in a subsequent process. Impediments in the complex wet-chemical production processes are the tendency thereof to form agglomerates and inclusions of gas, the complexity involved in providing the required purity, the drying and the solvents used and the binder residues incorporated within the surface, usually polymer chains, which adversely affect the electrical properties of the layer.
For the electrodes of the lithium ion batteries, the binder residues lead to unsatisfactorily high-ohm transfer resistances between the particles of the electrode layer. There are limits with regard to the layer thickness and/or the layer structure in the known PVD methods for surface upgrading or for production of porous active materials, such as sputtering or thermal evaporation, or they are too ineffective to achieve the required properties on the industrial scale.